Many cell types are highly asymmetric and produce prominent extensions (e.g. microvilli). This is common for epithelial cells (e.g. intestinal brush border cells), which typically produce them from their apical surface. In recent years, progress has been made in understanding the ways in which signaling pathways and the cytoskeleton promote the outgrowth of extensions. Rather less is known about how cells regulate the number of extensions and how the integrity of the extensions is maintained. Our long-term goal is to understand the mechanism cells use to insure the integrity of such extensions and control their number. We are studying this in the model organism Drosophila melanogaster, where we have used a genetic approach to identify genes whose function is essential for maintaining extension integrity. During the past period of support, we molecularly cloned two of these genes. One (tricornered) encodes the Drosophila NDR kinase and the other (furry) a large conserved protein. Phenotypic studies suggest that the products of these genes are not essential for the outgrowth of the extension, but instead they seem likely to participate in a novel process that insures extension integrity. The data suggests they could function as part of a monitoring pathway that senses morphogenetic damage and provides for repairs or that they could function in coordinating the assembly of various components of the extensions. During the next funding period, we propose to test a number of models for the function of the Tricornered kinase during morphogenesis. Mutant genes will be analyzed in transgenic animals to determine if the protein functions in the cytoplasm or nucleus and if it is activated by phosphorylation. We propose to carryout genetic screens to identify and study additional genes that function in the same process as tricornered The identity of these genes should provide insights into the tricornered dependent process. We also propose experiments to determine if tricornered mutations alter the responses of differentiating cells to actin inhibitors, if cellular damage alters the subcellular location or phosphorylation of Tricornered, and to determine the relationship between tricornered and furry.